Stability, ease of application, and reproducibility in manufacture of cosmetic preparations depend primarily on the rheological (fluid) properties of their components. Most cosmetic preparations contain gels which have a controlling influence on the rheological characteristics of the finished products. An important characteristic of cosmetic gels is their thixotropy. If thixotropic gels are subjected to increased shear forces, the viscosity of such gels is noticeably reduced. For example, when a shear force is applied, an apparently solid thixotropic substance becomes liquid for a period of time, but the original, solid gel structure slowly develops again once the shear force treatment ceases. This effect has wide utility in cosmetic and other preparations, such as in antiperspirants, creams, and nail enamels, but also in paints, inks and soaps.
Cosmetic gels are generally composed of a swellable argillaceous mineral which, upon taking up liquid, swells and forms a highly viscous gel. If the laminar structure of the argillaceous mineral is to be swelled by organic liquids, such as oils, fats and waxes, the argillaceous mineral must be modified to be organophilic or lipophilic.
Prior gel compositions contain argillaceous minerals having a layer structure, such as bentonite or montmorillonite, with a composition corresponding to the following formula: EQU (X, Y).sub.2-3 (Si,Al).sub.4 O.sub.10 (OH).sub.2 Z.sub.1/3 n H.sub.2 O
where X=Al, Fe(III), Mn(III) or Cr(III); Y=Mg, Fe(II), Mn(II), Ni, Zn or Li and Z=K, Na or Ca.
Such a silicate is hydrophilic and is able to swell while absorbing a large quantity of water between its lattice planes to form aqueous gels having a high viscosity.
It is also known that an organophilic or lipophilic material can be produced from such a hydrophilic substance by ion exchange. For example, Na.sup.+ ions in the original material can be replaced by quaternary ammonium cations having long-chain organic groups (see J. W. Jordan, Jour. Phys. and Colloid Chem. 53, p. 294 (1949), and J. W. Jordan et al, Kolloid Z, 137 p. 40, (1954) or U.S. Pat. Nos. 4,724,098 and 4,434,076). Materials produced in this manner are called "organically modified." These organically modified argillaceous minerals exhibit a good swelling effect in oils, fats and waxes and form viscous gels with them if mechanical energy is applied, suitable additives are added and the temperature is suitable.
A gel composition is also known which includes an organically modified or unmodified argillaceous mineral of the montmorillonite series, a surfactant and an organic solvent (F. Gstirner, Grundstoffe und Verfahren der Arzneimittelzubereitung [Basic Ingredients and Methods of Pharmaceutical Preparation] page 715, 1960). However, the gel forming ability of these montmorillonites is very low so that large quantities of expensive montmorillonite must be used. However, when pigments are added to these compositions, the pigments separate easily and the desired viscosity of the gel preparation is not readily maintained.
Other gel compositions include 10% organically modified montmorillonite, 86.7% mineral oil and 3.3% wetting agent (also called a polar additive; H. P. Fiedler, Lexikon der Hilfsstoffe [Encyclopedia of Auxiliary Substances], EDITIO Cantor Aulendorf [Cantor Aulendorf Edition], page 167).
A particular drawback of prior gel preparations is that the high percentages of organic polar additives employed, such as methanol, ethanol, and acetone, and the quaternary ammonium salts introduced by ion exchange, have a skin-irritating effect and may even cause allergic reactions. Moreover, the impurities of the montmorillonite minerals and the various organic additives produce a yellow to brown coloration and an unpleasant odor. These are especially disadvantageous when the preparations are to be used in cosmetics.